Six years ago, when Mayor Richard Daley had the roof installed, it was an oddity. Today, more than 200 green roofs in the city have been constructed or are under way, covering some 2-1/2 million square feet of tar with plants – by far the most of any American city.

Now other cities, hoping to cool and clean their air and help with storm drainage, are beginning to emulate Chicago, and the city is taking key steps to encourage – and in some cases require – private developers to follow City Hall's example.

Chicago's City Council just announced a pilot program that will provide up to $100,000 in matching funds for developers who retrofit existing downtown buildings with green roofs, out of a $500,000 pool of financing. Last year, the city began awarding small $5,000 grants to smaller projects, many residential. A green permitting process is designed to expedite requests. And Chicago has started requiring green roofs on new buildings that receive city financing.

"You look down on the prime real estate areas of this country – downtown Chicago, Manhattan – and so much is unutilized, all these rooftops," says Sadhu Johnston, Chicago's environment commissioner. The green-roof push, he says, is just one piece of a larger plan for the city that has included adding hundreds of thousands of trees, increasing energy efficiency, and replacing some traffic lanes with planted medians. "It's about a comprehensive strategy of making Chicago a better place to live."

Green roofs may be surprising in a city still more known for manufacturing than composting, but they are relatively common in Europe. Germany – the country that gave Mayor Daley the idea – has green roofs on about 20 percent of all flat roofs, according to one estimate. With a history dating back to the hanging gardens of Babylon, green roofs range from simple trays filled with hardy plants like sedum, to complex systems like the City Hall roof, which features some 150 plant species, a small apiary, and two trees.

In Chicago, they sit atop the Apple store, a Target, and a McDonald's. Even Chicago's soon-to-open Wal-Mart will have one – the company's first.

Nationally, green roofs grace the Gap headquarters in San Bruno, Calif.; a Ford Motor plant in Dearborn, Mich.; and the American Society of Landscape Architects building in Washington.

The idea is simple: bring back some of the organic material displaced by buildings, streets, and parking lots. Advocates tout benefits that range from reducing the urban "heat island" effect – which makes cities several degrees warmer than surrounding areas and can translate into millions of dollars in energy costs – to lengthening the life span of a roof, providing community garden or recreation space, and contributing to a building's energy efficiency.

"Cities are just going to keep getting hotter," says Steven Peck, president of Green Roofs for Healthy Cities. "So you take away hot surfaces and turn them into air conditioners. Green roofs do that very, very well."

On City Hall, for instance, the ambient temperature on the planted, city side of the roof is often 50 to 70 degrees cooler than that on the county side, still traditional black tar. Commissioner Johnston acknowledges that it's hard to know how many such roofs are needed before the effects become felt throughout the city, but he's determined to keep encouraging them until Chicago gets there.

"It's like turning off the water when you brush your teeth," he says. "Every building that does it this way has an effect."

This is why Chicago is doing its best to push private developers to follow City Hall's example. Every new roof in the city is already required to be reflective – another step to minimize urban heat island – but the latest matching-funds initiative is designed to show existing buildings that they, too, can establish green roofs.

"There are certain preconceived notions that it's easier to do it with new construction than with existing construction," says Constance Buscemi, spokeswoman for the city's Office of Planning and Development.

But there can be drawbacks. It's often twice as expensive to install a green roof, though experts say that's usually recouped through the roof's lengthened life span (they can last 40 or 50 years instead of the typical 20 or 25) and energy savings for the building. And some buildings simply aren't designed for the additional load, even when that's just a few inches of lightweight soil.

In a recent survey by Green Roofs for Healthy Cities, Chicago was followed by Washington and Suitland, Md. (home of a huge green-topped National Oceanic and Atmospheric Administration building) in green-roof square footage. But the amount of space is increasing rapidly – up 80 percent in the US between 2004 and 2005.

"What we've seen in Europe is that once the technology was understood and people saw that it worked, combined with incentives from the regulatory side, it really blossomed as an idea," says David Yocca, a senior partner at the Conservation Design Forum in Elmhurst, Ill., who has designed a number of green roofs. "It's the sort of idea that makes a lot of sense."